Technique to regulate an efficiency of a fuel cell system

ABSTRACT

A technique that is usable with a fuel cell stack includes providing a fuel flow to the stack, changing the fuel flow and observing a response of at least one cell voltage of the stack to the change in the fuel flow. An efficiency of the stack is regulated based on the observation.

BACKGROUND

[0001] The invention generally relates to a technique to regulate anefficiency of a fuel cell system.

[0002] A fuel cell is an electrochemical device that converts chemicalenergy produced by a reaction directly into electrical energy. Forexample, one type of fuel cell includes a polymer electrolyte membrane(PEM), often called a proton exchange membrane, that permits onlyprotons to pass between an anode and a cathode of the fuel cell. Themembrane is sandwiched between an anode catalyst layer on one side, anda cathode catalyst layer on the other side. This arrangement is commonlyreferred to as a membrane electrode assembly (MEA). At the anode,diatomic hydrogen (a fuel) is reacted to produce hydrogen protons thatpass through the PEM. The electrons produced by this reaction travelthrough circuitry that is external to the fuel cell to form anelectrical current. At the cathode, oxygen is reduced and reacts withthe hydrogen protons to form water. The anodic and cathodic reactionsare described by the following equations:

H₂→2H⁺+2e ⁻ at the anode of the cell, and

O₂+4H⁺+4e ⁻ →2 H₂O at the cathode of the cell.

[0003] A typical fuel cell has a terminal voltage near one volt DC. Forpurposes of producing much larger voltages, several fuel cells may beassembled together to form an arrangement called a fuel cell stack, anarrangement in which the fuel cells are electrically coupled together inseries to form a larger DC voltage (a voltage near 100 volts DC, forexample) and to provide more power.

[0004] The fuel cell stack may include flow plates (graphite compositeor metal plates, as examples) that are stacked one on top of the other,and each plate may be associated with more than one fuel cell of thestack. The plates may include various surface flow channels and orificesto, as examples, route the reactants and products through the fuel cellstack. Several PEMs (each one being associated with a particular fuelcell) may be dispersed throughout the stack between the anodes andcathodes of the different fuel cells. Electrically conductive gasdiffusion layers (GDLs) may be located on each side of each PEM to formthe anode and cathodes of each fuel cell. In this manner, reactant gasesfrom each side of the PEM may leave the flow channels and diffusethrough the GDLs to reach the PEM. The PEM and its adjacent pair areoften assembled together in an arrangement sometimes called a membraneelectrode unit (MEU).

[0005] A fuel cell system may include a fuel processor that converts ahydrocarbon (natural gas, propane methanol, as examples) into the fuelfor the fuel cell stack. For a given output power of the fuel cellstack, the fuel and oxidant flow to the stack must satisfy theappropriate stoichiometric ratios governed by the equations listedabove. Thus, a controller of the fuel cell system may monitor the outputpower of the stack and based on the monitored output power, estimate thefuel and air flow to satisfy the appropriate stoichiometric ratios. Inthis manner, the controller regulates the fuel processor to produce thisflow, and in response to the controller detecting a change in the outputpower, the controller estimates a new rate of fuel and air flow andcontrols the fuel processor accordingly.

[0006] Due to nonideal characteristics of the stack, it may be difficultto precisely predict the rate of fuel and air flow needed for a givenoutput power. Therefore, the controller may build in a sufficient marginof error by causing the fuel processor to provide more fuel and/or airthan is necessary to ensure that the cells of the stack receive enoughfuel and thus, are not “starved” for fuel or air. However, such acontrol technique may be quite inefficient, as the fuel cell stacktypically does not consume all of the incoming fuel, leaving unconsumedfuel that may burned off by an oxidizer of the fuel cell system.

[0007] Thus, there is a continuing need for an arrangement and/ortechnique to address one or more of the problems that are recited above.

SUMMARY

[0008] In an embodiment of the invention, a technique that is usablewith a fuel cell stack includes providing a fuel flow to the stack,changing the fuel flow and observing a response of at least one cellvoltage of the stack to the change in the fuel flow. An efficiency ofthe stack is regulated based on the observation.

[0009] Advantages and other features of the invention will becomeapparent from the following description, from the drawing and from theclaims.

BRIEF DESCRIPTION OF THE DRAWING

[0010]FIG. 1 is a schematic diagram of a fuel cell system according toan embodiment of the invention.

[0011]FIGS. 2, 3, 4, 5, 6 and 8 are flow diagrams depicting techniquesto regulate an efficiency of the fuel cell system according toembodiments of the invention.

[0012]FIG. 7 is an illustration of cells of the fuel cell stackaccording to an embodiment of the invention.

[0013]FIG. 9 is a schematic diagram of a cell voltage monitoring circuitof the system of FIG. 1 according to an embodiment of the invention.

DETAILED DESCRIPTION

[0014] Referring to FIG. 1, an embodiment of a fuel cell system 10 inaccordance with the invention includes a fuel cell stack 20 that iscapable of producing power for a load 50 (a residential load, forexample) in response to fuel and oxidant flows that are provided by afuel processor 22 and an air blower 24, respectively. In this manner,the fuel cell system 10 controls the fuel production of the fuelprocessor 22 to control the rate at which fuel is provided to the fuelcell stack 20. As described below, the fuel cell system 10 bases (atleast in part) its regulation of the fuel processor 22 on measured cellvoltages of the fuel cell stack 20, as the system 10 uses one or more ofthese measured cell voltages as an indicator of how efficiently the fuelcell system 10 is running.

[0015] Referring also to FIG. 2, more specifically, in some embodimentsof the invention, the system 10 uses a technique 70 to control theefficiency of the fuel cell stack 20 with respect to the fuel flow. Inthe technique 70, a fuel flow is provided (block 72) to the fuel cellstack 20 and the rate of the flow is changed (block 74). The response ofat least one cell voltage to this change is observed (block 76), and theefficiency of the fuel cell stack 20 is regulated (block 78) based onthis observation. Because the output power of the fuel cell stack 20 maychange over time and because the behavior of the stack 20 itself maychange over time, the technique 70 may include returning to block 74 forpurposes of continually performing blocks 74, 76 and 78 in a loop.

[0016] In some embodiments of the invention, the fuel cell system 10includes a cell voltage monitoring circuit 40 (see FIG. 1) to measurethe cell voltages and communicate (via a serial bus 48, for example)indications of the measured cell voltages to a controller 60 of thesystem 10. The controller 60 executes a program 65 (stored in a memory63 of the controller 60) to use the measured voltages to control thefuel processor 22 to perform the technique 70. In this manner, theexecution of the program 65 may, in some embodiments of the invention,cause the controller 60 to perform a routine 100 that is depicted inFIG. 3.

[0017] Referring to FIGS. 1 and 3, the routine 100 may be initiated, forexample, after the fuel cell system 10 has powered up from a shut downstate. In the routine 100, the controller 60 pinpoints the rate of fuelflow that, for the given output power of the fuel cell stack 20,satisfies the appropriate stoichiometric ratios and does not produce asignificant amount of unconsumed fuel. Therefore, the routine 100maximizes the efficiency of the fuel cell stack 20 with respect to thefuel flow.

[0018] In the routine 100, the controller 60 regulates the fuelprocessor 22 to decrease the fuel flow to the stack 20 by apredetermined amount, as depicted in block 102 of FIG. 3. Thispredetermined amount may be a fixed amount or may be a predeterminedpercentage of the current flow rate, as just a few examples. Next, thecontroller 60 obtains (block 104) indications of the cells voltages. Asan example, in some embodiments of the invention, the cell voltagemonitoring circuit 40 may provide indications of the most recentlymeasured cell voltages to the controller 60 via the serial bus 48.Depending on the particular embodiment of the invention, the cellvoltage monitoring circuit 40 may provide the indications of thevoltages when requested by the controller 60 or may periodically providethe indications, as just a few examples.

[0019] After the controller 60 receives the indications of the cellvoltages, the controller 60 determines (diamond 106) from the cellvoltages whether the efficiency of the fuel cell stack 20 with respectto the fuel flow can be improved. In this manner, in some embodiments ofthe invention, if the cell voltages indicate that, after the decrease infuel flow, the fuel cell stack 20 is receiving a sufficient amount offuel, control returns to block 102 to decrease the flow again.Otherwise, the controller 60 has pinpointed a fuel flow to maximizeefficiency and regulates the fuel processor 22 to increase (block 108)the fuel flow by a predetermined amount to return the rate of fuel flowback to the level that existed before the last decrease. For example, ifthe controller 60 decreases the fuel flow by 5.00 percent andsubsequently determines the efficiency cannot be improved in response toobserving the cell voltages' response, the controller 60 increases thefuel flow by 5.26 percent to return the fuel flow to the level beforethe decrease. Other rates of increase and/or decrease may be used.

[0020] After increasing the fuel flow, the controller 60 subsequentlydelays (block 110) for a predetermined time interval (one to fiveminutes, for example) before returning to block 102. The return to block102 is needed to accommodate potentially changing operating conditionsdue to the aging of stack 20, variations in the power that is demandedby the load 50, etc.

[0021] It is noted that other control loops may be used in combinationwith the routine 100. For example, the controller 60 may adjust the fuelflow in response to a monitored output power of the fuel cell stack 20.However, the controller 60 still maintains the control provided by theroutine 100 to improve the efficiency of the fuel cell stack 20 withrespect to the fuel flow.

[0022] In some embodiments of the invention, circuitry other than thecontroller 60 may be used to perform one or more parts of the routine100. For example, in some embodiments of the invention, the cell voltagemonitoring circuit 40 may determine whether the efficiency can beimproved and indicate to the controller 60 whether to increase ordecrease the fuel flow based on this determination. For purposes ofsimplifying the description below, it is assumed that the controller 60determines whether the efficiency can be improved, although othervariations are possible.

[0023] There are numerous ways for the controller 60 to determinewhether the efficiency can be improved. For example, FIG. 4 depicts aroutine 120 that the controller 60 may perform (when executing theprogram 65) to make this determination. In the routine 120, thecontroller 60 retrieves an indication of, or reads, the cells voltagesone at a time to determine if one of the cell voltages indicates thatthe corresponding cell is being deprived of sufficient fuel. In thismanner, the controller 60 reads (block 122) the next cell voltage thatis provided by the cell voltage monitoring circuit and compares (block124) the cell voltage to a predetermined threshold (a voltage between−0.5 and 0.5 volts, as an example). When a cell of the fuel cell stack20 is starved of fuel, the voltage of the cell significantly drops, andthe detection of this drop is provided by the comparison of the cellvoltage to the predetermined threshold. Thus, if the controller 60determines (diamond 126) that the cell voltage is below thepredetermined threshold, control returns to block 108 (see FIG. 3) ofthe routine 100. Otherwise, the controller 60 determines (diamond 128)if all cell voltages have been read. If not, the controller 60 reads(block 122) the next cell voltage. If all cell voltages have been read,control returns to block 102 (see FIG. 3) of the routine 100.

[0024] In some embodiments, a fuel flow limit may be set on the fuelflow that could be used to sustain the cells within the acceptablevoltage range. For example, when a cell voltage remains under thepredetermined voltage threshold after the fuel flow has been increasedto such a limit, the fuel cell system may be programmed to shut itselfoff or activate a low efficiency signal or alarm, as examples. In otherembodiments, when the fuel flow limit is reached, the system can resetthe fuel flow and then similarly increase the oxidant flow to see if thelow cell voltage can be brought above the desired threshold. The fueland oxidant flows can also be manipulated at the same time. Otherembodiments are also possible.

[0025]FIG. 5 depicts an alternative routine 140 that the controller 60may use to determine (diamond 106 of FIG. 3) if the efficiency of thefuel cell stack 20 with respect to the fuel flow can be improved. In theroutine 140, the controller 60 reads the cell voltages one at a time todetermine if one of the cell voltages indicates that the correspondingcell is being deprived of sufficient fuel. However, unlike the routine100, in the routine 140, the controller 60 permits a certain number ofthe cell voltages to fall below the predetermined voltage threshold.

[0026] In this manner, in the routine 140, the controller 60 reads(block 142) the next cell voltage that is provided by the cell voltagemonitoring circuit 40 and compares (block 144) the cell voltage to thepredetermined cell voltage threshold. If the controller 60 determines(diamond 146) that the cell voltage is below the predeterminedthreshold, then the controller 60 determines (diamond 149) whether apredetermined number (a number between two to ten, as example) of cellvoltages have decreased below the threshold. If so, control returns toblock 108 (see FIG. 3) of the routine 100. Otherwise, control transfersto diamond 148, the same point where control is transferred if thecontroller 60 determines (diamond 146) that the cell voltage is notbelow the cell voltage threshold. In diamond 148, the controller 60determines if all cell voltages have been read. If so, control transfersto block 102 of the routine 100. Otherwise, control returns to block 142where the controller 60 reads the next cell voltage.

[0027]FIG. 6 depicts another routine 160 that the controller 60 may useto determine (diamond 106 of FIG. 3) of the routine 100) if theefficiency of the fuel cell stack 20 can be improved. In the routine160, the controller 60 reads all of the cell voltages that are providedby the cell voltage monitoring circuit 40. Next, the controller 60determines (block 164) a standard deviation between the cell voltages.If the controller 60 determines (diamond 166) that the standarddeviation is above a predetermined standard deviation threshold, thencontrol transfers to block 108 of the routine 100. Otherwise, controltransfers to block 102 of the routine 100. In other embodiments, otherindications may be used in lieu of standard deviation. For example, asfuel stoichiometry is reduced, some “weak” cells within a stack willtypically exhibit fuel starvation symptoms (e.g., voltage drop) morequickly than the rest of the cells in the stack. As the fuelstoichiometry is reduced, the voltage drop exhibited by such cells mayincrease exponentially, or at least at a greater rate than other cellsin the stack. Thus, the relative voltage drop of a particular cell withrespect to a given fuel reduction may also provide a measure accordingto which control may be transferred between blocks 102 and 108 of theroutine 100 (by comparing to such a measure for the other cells, or to apredetermined threshold, as examples). In some embodiments of theinvention, the efficiency may be controlled based on a subset of thecells of the fuel cell stack 20. In this manner, referring to FIG. 7,the fuel cell stack 20 may include cells 25 that are not monitored forpurposes of regulating the efficiency and a subset 25 of one or morecells that are monitored to regulate the efficiency.

[0028] The one or more cells of the subset 25 may be, in someembodiments of the invention, specially constructed so that theirvoltages decrease below the predetermined cell voltage threshold beforethe other cells 23. For example, the flow plates that are associatedwith the subset 25 may have fuel flow channels that are more narrow incross section than the channels for the other cells 23, and/or the flowplates that are associated with the subset 25 may have fewer fuel flowchannels than the flow plates that are associated with the other cells23. These modifications decrease the flow of fuel into the subset 25, ascompared to the other cells. Therefore, the voltages of the one or morecells of the subset 25 may be more sensitive to a decrease in fuel thanthe voltages of the other cells 23.

[0029] Thus, any of the techniques described above may be used with thecell(s) of the subset 25. For example, FIG. 8 depicts a routine 170 thatmay be used in place of the routine 100 in the case where the subset 25includes a single cell. In the routine 170, the controller 60 decreasesthe fuel flow to the stack 20 by a predetermined amount, as depicted inblock 172. Next, the controller 60 obtains (block 174) an indication ofthe voltage of the cell 25. Subsequently, the controller 60 determines(diamond 176) from the cell voltage whether the efficiency of the stackwith respect to the fuel flow can be improved. The controller 60 mayaccomplish this using one of the techniques that are described above.

[0030] If the cell voltage indicates that, after the decrease in fuelflow, the fuel stack 20 is receiving a sufficient amount of fuel,control returns to block 172 to decrease the flow again. Otherwise, thecontroller 60 has pinpointed the correct fuel flow for efficiency andincreases (block 178) the fuel flow by a predetermined amount to returnthe rate of fuel flow back to the level that existed before the lastdecrease. The controller 60 subsequently delays (block 180) for apredetermined time interval before control returns to block 172.

[0031] Referring back to FIG. 1, among the other features of the fuelcell system 20, the system 20 may include a voltage regulator 30 thatregulates a VTERM stack voltage (a DC voltage that is provided by a mainoutput terminal 31 of the fuel cell stack 20) and converts this voltageinto an AC voltage via an inverter 33. The output terminals 32 of theinverter 33 are coupled to the load 50. The fuel cell system 10 alsoincludes control valves 44 that provide emergency shutoff of the oxidantand fuel flows to the fuel cell stack 20. The control valves 44 arecoupled between inlet fuel 37 and oxidant 39 lines and the fuel andoxidant manifold inlets, respectively, to the fuel cell stack 20. Theinlet fuel line 37 receives the fuel flow from the fuel processor 22,and the inlet oxidant line 39 receives the oxidant flow from the airblower 24.

[0032] The fuel cell system 20 may include water separators, such aswater separators 34 and 36, to recover water from the outlet fuel andoxidant ports of the stack 22. The water that is collected by the waterseparators 34 and 36 may be routed to a water tank (not shown) of acoolant subsystem 54 of the fuel cell system 10. The coolant subsystem54 circulates a coolant (de-ionized water, for example) through the fuelcell stack 20 to regulate the operating temperature of the stack 20. Thefuel cell system 10 may also include an oxidizer 38 to burn any fuelfrom the stack 22 that is not consumed in the fuel cell reactions.

[0033] To monitor the power output of the fuel cell stack 20, the fuelcell system 10 may include a current sensing element 49 that is coupledin series between the main output terminal 31 of the stack 20 and theinput terminal of the voltage regulator 30. An electrical communicationline 52 provides an indication of the sensed current to the controller60. In this manner, the controller 60 may use the indications of cellvoltages and the stack voltage from the cell voltage monitoring circuit40 as well as the indication of the output current provided by thecurrent sensing element 49 to determine the output power of the fuelcell stack 20.

[0034] For purposes of isolating the load from the fuel cell stack 20during a shut down of the fuel cell system 10, the system 10 may includea switch 29 (a relay circuit, for example) that is coupled between themain output terminal 31 of the stack 20 and an input terminal of thecurrent sensing element 49. The controller 60 may control the switch 29via an electrical communication line 50.

[0035] In some embodiments of the invention, the controller 60 mayinclude a microcontroller and/or a microprocessor to perform one or moreof the routines described above when executing the program 65. Forexample, the controller 60 may include a microcontroller that includes aread only memory (ROM) that serves as the memory 63 and a storage mediumto store instructions for the program 65. Other types of storage mediumsmay be used to store instructions of the program 65. Various analog anddigital external pins of the microcontroller may be used to establishcommunication over the electrical communication lines 46, 50 and 52 andthe serial bus 48. In other embodiments of the invention, a memory thatis fabricated on a separate die from the microcontroller may be used asthe memory 63 and store instructions for the program 65. Othervariations are possible.

[0036]FIG. 9 depicts the cell voltage monitoring circuit 40 according toan embodiment of the invention. The cell voltage monitoring circuit 40includes voltage scanning units 200, each of which is associated withand measures the voltages of a different group of the cells. In thismanner, electrical communication lines 202 may connect the voltagescanning units 200 to the various terminals of the fuel cell stack 20.The ground of each voltage scanning unit 200 may be referenced to aterminal of the associated group of cells, as described in U.S. Pat. No.6,140,820, entitled, “MEASURING CELL VOLTAGES OF A FUEL CELL STACK,”granted on Oct. 31, 2000.

[0037] In some embodiments of the invention, the cell voltage monitoringcircuit 40 may include communication lines 206 that communicateindications of the measured cell voltages from the cell voltagemonitoring units 200 to an interface 207. The interface 207 may becoupled to a bus 212 that, in turn, may be coupled to a memory 214 thatstores data that indicates the measured voltages. A controller 208 ofthe cell voltage monitoring circuit 40 may execute a program 210 tocause the controller 208 to periodically cause the cell voltagemonitoring units 200 to measure the cell voltages, cause the memory 214to store the data that indicates the measured voltages, and cause aserial bus interface 220 to communicate indications of the measuredvoltages to the controller 60 via the serial bus 48.

[0038] While the invention has been disclosed with respect to a limitednumber of embodiments, those skilled in the art, having the benefit ofthis disclosure, will appreciate numerous modifications and variationstherefrom. It is intended that the appended claims cover all suchmodifications and variations as fall within the true spirit and scope ofthe invention.

What is claimed is:
 1. A method usable with a fuel cell stack,comprising: providing a fuel flow to the stack; changing the fuel flow;observing a response of at least one cell voltage of the stack to thechanging of the fuel flow; and regulating an efficiency of the stackbased on the observation.
 2. The method of claim 1, wherein the changingthe fuel flow comprises: decreasing the fuel flow by a predeterminedamount.
 3. The method of claim 2, wherein the predetermined amountcomprises a predetermined percentage of the fuel flow before thedecrease.
 4. The method of claim 1, wherein the observing comprises:measuring said at least one cell voltage of the stack, said at least onecell voltage being associated with at least one cell designed be moresensitive to the change in fuel flow than cells of the stack other thansaid at least one cell.
 5. The method of claim 1, wherein the observingcomprises: measuring said at least one cell voltage of the stack afterthe change in fuel flow.
 6. The method of claim 5, wherein the observingfurther comprises: determining if one of said at least one cell voltageis below a predetermined cell voltage threshold.
 7. The method of claim6, wherein the regulating comprises: increasing the fuel flow if the onecell voltage is below the predetermined cell voltage threshold.
 8. Themethod of claim 6, wherein the regulating comprises: repeating thedecreasing of the fuel flow until the one cell voltage is below thepredetermined cell voltage threshold.
 9. The method of claim 1, whereinthe observing comprises: determining if a predetermined number of saidat least one cell voltage is below a predetermined cell voltagethreshold.
 10. The method of claim 9, wherein the regulating comprises:increasing the fuel flow if the predetermined number of said at leastone cell voltage is below the predetermined cell voltage threshold. 12.The method of claim 9, wherein the regulating comprises: repeating thedecreasing of the fuel flow until the predetermined number of said atleast one cell voltage is below the predetermined cell voltagethreshold.
 13. The method of claim 1, wherein the observing comprises:determining a standard deviation of said at least one cell voltage. 14.The method of claim 13, wherein the regulating comprises: increasing thefuel flow if the standard deviation is above a predetermined threshold.15. The method of claim 13, wherein the regulating comprises: repeatingthe decreasing of the fuel flow until the standard deviation is above apredetermined threshold.
 16. A system comprising: a fuel cell stack; afuel processor to provide a fuel flow to the stack; and a first circuitcoupled to the fuel processor and the stack to: control the fuelprocessor to change the fuel flow, observe a response of at least onecell voltage of the stack to the changing of the fuel flow, and controlthe fuel processor to regulate an efficiency of the stack based on theobservation.
 17. The system of claim 16, wherein the first circuitcontrols the fuel processor to change the fuel flow by decreasing thefuel flow by a predetermined amount.
 18. The system of claim 16, furthercomprising: a second circuit to measure said at least one cell voltage,wherein the fuel cell stack comprises at least one cell associated withsaid at least one cell voltage, said at least one cell designed be moresensitive to the change in fuel flow than cells of the stack other thansaid at least one cell, and the first circuit is coupled to the secondcircuit to receive an indication of said at least one cell voltage. 19.The system of claim 16, further comprising: a second circuit to measuresaid at least one cell voltage.
 20. The system of claim 19, wherein thefirst circuit determines if one of the said at least one cell voltage isbelow a predetermined cell voltage threshold.
 21. The system of claim20, wherein the first circuit controls the fuel processor to increasethe fuel flow if the one cell voltage is below the predetermined cellvoltage threshold.
 22. The system of claim 20, wherein the first circuitcontrols the fuel processor to keep decreasing of the fuel flow untilthe one cell voltage is below the predetermined cell voltage threshold.23. The system of claim 16, further comprising: a second circuit coupledto the stack to provide an indication of said at least one cell voltageto the first circuit, wherein the first circuit uses the indication todetermine if a predetermined number of said at least one cell voltage isbelow a predetermined cell voltage threshold.
 24. The system of claim23, wherein the first circuit controls the fuel processor to increasethe fuel flow if the predetermined number of said at least one cellvoltage is below the predetermined cell voltage threshold.
 25. Thesystem of claim 23, wherein the first circuit controls the fuelprocessor to keep decreasing the fuel flow until the one cell voltage isbelow the predetermined cell voltage threshold.
 26. The system of claim16, further comprising: a second circuit coupled to the stack and thefirst circuit to measure said at least one cell voltage, wherein thefirst circuit determines a standard deviation of said at least one cellvoltage.
 27. The system of claim 26, wherein the first circuit controlsthe fuel processor to increase the fuel flow if the standard deviationis above a predetermined threshold.
 28. The system of claim 23, whereinthe first circuit controls the fuel processor to keep decreasing thefuel flow until the standard deviation is above a predeterminedthreshold.
 29. An article comprising a computer readable storage mediumstoring instructions to cause a computer to: interact with a fuelprocessor to change a rate at which the fuel processor is providing fuelto a fuel cell stack, observe a response of at least one cell voltage ofthe stack to the changing of the fuel flow; and control the fuelprocessor to regulate an efficiency of the stack based on theobservation.
 30. The article of claim 29, wherein the storage mediumstores instructions to cause the computer to decrease the rate by apredetermined amount.
 31. The article of claim 29, wherein the storagemedium stores instructions to cause the computer to use a cell voltagemonitoring circuit to measure said at least one cell voltage.
 32. Thearticle of claim 29, wherein the storage medium stores instructions tocause the computer to determine if one of the said at least one cellvoltage is below a predetermined cell voltage threshold and base theregulation of the efficiency of the stack on the determination.
 33. Thearticle of claim 29, wherein the storage medium stores instructions tocause the computer to determine if a predetermined number of said atleast one cell voltage is below a predetermined cell voltage thresholdand base the regulation of the efficiency of the stack on thedetermination.
 34. The article of claim 29, wherein the storage mediumstores instructions to cause the computer to determine a standarddeviation of said at least one cell voltage and base the regulation ofthe efficiency on the determination.
 35. An apparatus usable with a fuelprocessor and a fuel cell stack providing cell voltages, comprising: afirst circuit to measure at least one of the cell voltages; and a secondcircuit coupled to the first circuit, the fuel cell stack and the fuelprocessor to: control the fuel processor to change the fuel flow,observe a response of at least one cell voltage of the stack to thechanging of the fuel flow, and control the fuel processor to regulate anefficiency of the stack based on the observation.
 35. The apparatus ofclaim 35, wherein the second circuit controls the fuel processor tochange the fuel flow by decreasing the fuel flow by a predeterminedamount.
 36. The apparatus of claim 35, wherein the second circuitdetermines if one of the said at least one cell voltage is below apredetermined cell voltage threshold and controls the fuel processor toregulate an efficiency of the stack based on the determination.
 37. Theapparatus of claim 35, wherein the second circuit determines if apredetermined number of said at least one cell voltage is below apredetermined cell voltage threshold and controls the fuel processor toregulate an efficiency of the stack based on the determination.
 38. Theapparatus of claim 35, wherein the second circuit determines if astandard deviation of said at least one cell voltage is above apredetermined threshold and controls the fuel processor to regulate anefficiency of the stack based on the determination.